The present invention relates generally to the frequency shifting of optical energy as generated by a laser, for example, and more particularly to the accomplishment of such shifting to specific frequencies and at high efficiency using a moving grating material.
The present invention concerns a device capable of converting multimode or multi-beam laser systems into a single mode beam with near 100% efficiency. In a line narrowing configuration of the invention, a multimode laser beam of equally spaced frequencies is sent into a nonlinear material device having either a positive or negative Kerr constant. In the device, the energy in each laser mode is shifted up to the highest frequency for a negative Kerr constant, or down to the lowest frequency for a positive Kerr constant. In the frequency shifting configuration, a multiline laser beam or several laser beams of different frequencies are made incident on the device. Positive or negative Kerr constants are utilized to shift the beams up or down the mode ladder to the highest or lowest frequency.
In the line narrowing configuration of the invention therefore, one could send a multimode laser beam of equally spaced frequencies into a transducer which includes a nonlinear material having either a positive or a negative Kerr constant characteristic. In the positive or negative Kerr constant material, the energy in each laser mode is shifted up (for a negative Kerr constant) or down (for a positive Kerr constant) the mode ladder to the highest or lowest frequency of the input spectrum.
In the frequency shifting configuration therefore, either a multiline laser beam or several different laser beams of different frequencies would be incident on the nonlinear material. Shifting up in frequency would require a material having a negative Kerr constant. As the beam or beams exit the device, near 100% output at the highest or lowest frequency can be obtained. If the required frequency is not the highest or lowest input mode, but lies somewhere between, a power conversion into these frequencies is also possible.
Near 100% total input to total output power conversion occurs from multimode operation to single mode operation in the present invention. In conventional line-narrowing devices, such as filters and etalons, the power not in the desired single mode is merely discarded. However, in the present invention nonlinear device, power in the unwanted modes is actually shifted into the desired mode. The same device can be used in a laser frequency-shifting configuration. This device can also convert multimode or multi-beam systems into a single mode beam with near-100% efficiency, thus providing an efficient laser frequency tuning capability.
Most high-power laser output is multimode in nature. The many modes in such devices, however, tend to interfere and degrade the propagation and focusing properties of the beam. The device of the present invention provides an efficient means of conversion of such multimode output power into a single, very high power laser mode having diffraction-limited propagation and focusing properties.
The patent art indicates the presence of significant inventive activity relating to optical frequency shifting. Included in this art is the patent of P. B. Scott, U.S. Pat. No. 4,897,843 which describes a high speed broadband tunable laser system with multiple tuning elements. The Scott tuning elements comprise individual birefringent crystals which exhibit electro-optic effects when an electric field is applied. Two or more such elements provide coarse and progressively finer control over the wavelength and linewidth. The birefringent crystals are cut so as to have an optical transmission path which is roughly aligned with the Z-axis of the crystal. Two or more electrodes are positioned at opposite ends of the crystal. When a voltage is applied to the crystal, a frequency modifying phenomenon is obtained.
Also included in this art is the patent of J. F. Weller et al, U.S. Pat. No. 4,503,541 which relates to a controlled linewidth laser source in which the linewidth is controlled by reflecting a certain portion of the laser light back into the laser cavity. In the Weller et al apparatus, the active layer on an electroabsorptive cell is aligned with the active layer of a semiconductor laser on a single substrate, with a first waveguide therebetween. The first waveguide guides light from the laser to the electroabsorptive cell. The light absorption of the cell is electronically controlled. The laser light propagates through the active region in the electroabsorptive cell, and then a second waveguide guides the laser light to a reflection facet. The reflection facet reflects the laser light back through the waveguide-cell-waveguide-laser light path. The amount of light fed back depends on the absorption in the electroabsorption cell, which depends on the voltage applied to the cell. In this way, electronic control is achieved over the reflection process.
Additionally included in this art is the patent of K. Guers et al, U.S. Pat. No. 4,264,141 which describes a method and apparatus for frequency shifting a monochromatic narrow bandwidth light beam by means of the Doppler effect. The Guers et al apparatus comprises two stationary mirrors and two moving mirrors. The moving mirrors are arranged symmetrically about their axis of rotation, and staggered at a fixed angle of 180.degree. on a rotating support. The beam is reflected back and forth between two locally fixed mirrors via the two moving mirrors. The directional alteration of the beam associated with the reflection from the first moving mirror is compensated by the reflection from the second moving mirror. The beam direction is reversed at the subsequent stationary mirror, and traverses the apparatus arrangement in the opposite direction and hits the surface of the second stationary mirror, and the beam is reversed again. This procedure is repeated for a number of light passages before the beam is coupled out of the apparatus.
Further included in this art is the patent of D. P. Hutchinson et al, U.S. Pat. No. 4,019,157 which relates to a method and apparatus for tuning high power lasers to a desired wavelength. The beam from a low power laser, which is lasing at the desired wavelength is injected into the high power laser through a small hole in the rear reflector of the high power laser. The injected signal from the low power laser overrides the spontaneous emission which normally starts laser oscillation, and locks the high power laser to the injection frequency.
Although these prior patents relate to systems and devices to control laser frequency, they do not describe a device which utilizes a nonlinear material with positive or negative Kerr constant to shift the laser frequency as described herein.